Conventionally, as for a fastener fastening the fastened member to the fastening member, there is, for example, Patent Document 1. In the conventional technology, a guide portion of a pin is inclined relative to a shaft line of the pin in two steps, and a partition on an end side of the guide portion becomes a first guide portion including a guide face inclined at 30 to 65 degrees to the shaft line of the pin, and a partition on a base end side of the guide portion becomes a second guide portion including a guide face inclined at 5 to 25 degrees to the shaft line of the pin. Then, in a first half of an insertion wherein the pin is pressed into a grommet, a guided portion is guided by the guide face of the first guide portion so as to expand a leg portion. Also, in a second half of the insertion, the guided portion is guided by the guide face of the second guide portion so as to expand the leg portion.